U.S. patent application number 13/366313 was filed with the patent office on 2012-09-13 for semiconductor device and its fabrication method.
This patent application is currently assigned to HITACHI, LTD.. Invention is credited to Takashi Ishigaki, Kazuhiro Mochizuki, Akihisa Terano, Tomonobu Tsuchiya.
Application Number | 20120228626 13/366313 |
Document ID | / |
Family ID | 46794721 |
Filed Date | 2012-09-13 |
United States Patent
Application |
20120228626 |
Kind Code |
A1 |
Mochizuki; Kazuhiro ; et
al. |
September 13, 2012 |
SEMICONDUCTOR DEVICE AND ITS FABRICATION METHOD
Abstract
In a semiconductor device including a stack structure having
heterojunction units formed by alternately stacking GaN (gallium
nitride) films and barrier films which are different in forbidden
band width, a first electrode formed in a Schottky barrier contact
with one sidewall of the stack structure, and a second electrode
formed in contact with the other sidewall, an oxide film is
interposed between the first electrode and the barrier films.
Therefore, the reverse leakage current is prevented from flowing
through defects remaining in the barrier films due to processing of
the barrier films, so that a reverse leakage current of a Schottky
barrier diode is reduced.
Inventors: |
Mochizuki; Kazuhiro; (Tokyo,
JP) ; Ishigaki; Takashi; (Hino, JP) ; Terano;
Akihisa; (Hachioji, JP) ; Tsuchiya; Tomonobu;
(Hachioji, JP) |
Assignee: |
HITACHI, LTD.
|
Family ID: |
46794721 |
Appl. No.: |
13/366313 |
Filed: |
February 4, 2012 |
Current U.S.
Class: |
257/76 ; 257/201;
257/E21.09; 257/E29.091; 438/478 |
Current CPC
Class: |
H01L 29/66143 20130101;
H01L 29/2003 20130101; H01L 29/205 20130101; H01L 29/0649 20130101;
H01L 29/872 20130101 |
Class at
Publication: |
257/76 ; 257/201;
438/478; 257/E29.091; 257/E21.09 |
International
Class: |
H01L 29/205 20060101
H01L029/205; H01L 21/20 20060101 H01L021/20 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 7, 2011 |
JP |
JP2011-048728 |
Claims
1. A semiconductor device comprising: a stack structure having at
least one layer of a heterojunction unit in which a first film and
a second film mutually different in forbidden band width are
hetero-joined and stacked on a substrate; a first electrode
arranged so as to contact a first sidewall of the stack structure
and forms a Schottky barrier contact with the first film; and a
second electrode arranged so as to contact a second sidewall
opposite to the first sidewall of the stack structure, a first
insulating film being interposed between the first electrode and
the second electrode.
2. The semiconductor device according to claim 1, wherein the first
film contains GaN (gallium nitride), and the second film contains
AlGaN (aluminum gallium nitride) or InAlGaN (indium aluminum
gallium nitride).
3. The semiconductor device according to claim 1, wherein the stack
structure has a plurality of the heterojunction units stacked on
the substrate.
4. The semiconductor device according to claim 1, wherein the first
insulating film contains an oxide of the second film.
5. The semiconductor device according to claim 1, wherein the
second electrode and the second film directly contact each other in
the second sidewall.
6. The semiconductor device according to claim 1, wherein the
second electrode and the first film are ohmic-connected each other
in the second sidewall.
7. The semiconductor device according to claim 1, wherein the first
electrode and the second electrode are formed so as to ride over
the stack structure, and a GaN (gallium nitride) film is interposed
between the first electrode and the second electrode and an upper
surface of the stack structure.
8. The semiconductor device according to claim 1, wherein the first
electrode and the second electrode are formed so as to ride over
the stack structure, and the second insulating film is interposed
between the first electrode and the second electrode and the upper
surface of the stack structure.
9. A fabrication method of a semiconductor device comprising the
steps of: (a) forming a stack structure by stacking alternately and
repeatedly a first film and a second film different in forbidden
band width on a substrate; (b) forming a first sidewall and a
second sidewall opposite to the first sidewall of the stack
structure by selectively removing the stack structure; (c) forming
a first insulating film on a side surface of the second film
exposed to the first sidewall; (d) forming a first electrode
contacting the first sidewall after the step (c), and forming a
Schottky barrier contact of the first film and the first electrode;
and (e) forming a second electrode contacting the second sidewall,
the first insulating film being interposed between the first
electrode and the second film.
10. The fabrication method of a semiconductor device according to
claim 9, wherein the first film contains GaN (gallium nitride), and
the second film contains AlGaN (aluminum gallium nitride) or
InAlGaN (indium aluminum gallium nitride).
11. The fabrication method of a semiconductor device according to
claim 9, wherein, in the step (c), the first insulating film
containing an oxide of the second film is formed by oxidizing a
side surface of the second film exposed to the first sidewall.
12. The fabrication method of a semiconductor device according to
claim 11, wherein, in the step (c), the first insulating film is
formed by oxidizing the second film by ozone radiation or wet
oxidization.
13. The fabrication method of a semiconductor device according to
claim 9, wherein, in the step (e), the first film and the second
electrode are ohmic-connected.
14. The fabrication method of a semiconductor device according to
claim 9, further comprising a step of forming a GaN (gallium
nitride) film on the stack structure before the step (b) after the
step (a), wherein the first electrode is formed on the GaN film in
the step (d); and the second electrode is formed on the GaN film in
the step (e).
15. The fabrication method of a semiconductor device according to
claim 9, further comprising a step of forming a second insulating
film on the stack structure before the step (b) after the step (a),
wherein the first electrode is formed on the second insulating film
in the step (d); and the second electrode is formed on the second
insulating film in the step (e).
16. The fabrication method of a semiconductor device according to
claim 15, wherein the second insulating film immediately below the
first electrode and the second insulating film immediately below
the second electrode are isolated from each other by partially
removing the second insulating film after the step (d) and the step
(e).
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application claims priority from Japanese Patent
Application No. 2011-048728 filed on Mar. 7, 2011, the content of
which is hereby incorporated by reference into this
application.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to a semiconductor device and
its fabrication method. The present invention particularly relates
to a technology effectively applied to a semiconductor device
having a heterojunction unit in which the widths of forbidden band
differ.
BACKGROUND OF THE INVENTION
[0003] A heterojunction unit of a barrier film and a GaN (gallium
nitride) film formed of In.sub.xAl.sub.yGa.sub.1-x-yN (indium
aluminum gallium nitride; 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1,
0.ltoreq.x+y.ltoreq.1) which is a nitride semiconductor has a high
breakdown electric field and a high sheet carrier concentration as
compared with a diode constituted by a p-n junction using silicon
or the like. Thus, it is proposed that, by using a diode (Schottky
barrier diode) for such a heterojunction unit, the breakdown
voltage is improved as compared with the diode composed of the p-n
junction such as silicon so that a diode performance such as
reduction of on-resistance is improved.
[0004] For example, Japanese Patent Application Laid-Open
Publication No. 2009-117485 (Patent Document 1) discloses a
Schottky barrier diode, which includes: a stack structure having a
heterojunction unit in which a GaN layer and an AlGaN (aluminum
gallium nitride) layer are stacked; a Schottky electrode that forms
a Schottky barrier contact with the heterojunction unit and is
formed on a first end of this stack structure; and an ohmic
electrode that forms an ohmic contact with the heterojunction unit
and is formed on a second end of this stack structure. However,
Patent Document 1 does not mention forming of an insulating film
such as an oxide film between the heterojunction unit and the
Schottky electrode.
SUMMARY OF THE INVENTION
[0005] The inventors of the present invention have studied
improvements of characteristics of a semiconductor device using a
heterojunction unit in which semiconductor films different in
forbidden band width are stacked for a diode and the like.
[0006] In conjunction with this study, the inventors have found
that, when the heterojunction unit having a stack structure is
processed (patterned by using dry etching), a semiconductor film
(for example, an AlGaN layer) exposed at a sidewall of the
heterojunction unit is apt to develop defects, thereby
deteriorating the characteristics of the diode. Specifically, a
leakage current is apt to be generated when a reverse bias voltage
is applied to the diode including the heterojunction unit, and this
raises a problem that a prescribed breakdown voltage cannot be
secured.
[0007] A preferred aim of the present invention is to prevent
generation of a reverse leakage current in the heterojunction
unit.
[0008] The above and other preferred aims and novel characteristics
of the present invention will be apparent from the description of
the present specification and the accompanying drawings.
[0009] The typical ones of the inventions disclosed in the present
application will be briefly described as follows.
[0010] A semiconductor device according to an invention in the
present application includes: a stack structure having at least one
layer of a heterojunction unit in which a first film and a second
film mutually different in forbidden bandwidth are hetero-joined
and stacked on a substrate; a first electrode arranged so as to
contact a first sidewall of the stack structure and forms a
Schottky barrier contact with the first film; and a second
electrode arranged so as to contact a second sidewall opposite to
the first sidewall of the stack structure, a first insulating film
being interposed between the first electrode and the second
electrode.
[0011] A fabrication method of a semiconductor device according to
an invention in the present application includes the steps of: (a)
forming a stack structure by stacking alternately and repeatedly a
first film and a second film different in forbidden band width on a
substrate; (b) forming a first sidewall and a second sidewall
opposite to the first sidewall of the stack structure by
selectively removing the stack structure; (c) forming a first
insulating film on a side surface of the second film exposed to the
first sidewall; (d) forming a first electrode contacting the first
sidewall after the step (c), and forming a Schottky barrier contact
of the first film and the first electrode; and (e) forming a second
electrode contacting the second sidewall, the first insulating film
being interposed between the first electrode and the second
film.
[0012] The effects obtained by typical aspects of the present
invention will be briefly described below.
[0013] According to the present invention, occurrence of a reverse
leakage current in a diode can be prevented.
BRIEF DESCRIPTIONS OF THE DRAWINGS
[0014] FIG. 1 is a plan view of a Schottky barrier diode that is a
first embodiment of the present invention;
[0015] FIG. 2 is a perspective view showing a partial cutaway view
of the Schottky barrier diode that is the first embodiment of the
present invention;
[0016] FIG. 3 is a cross-sectional view showing a fabrication
method of the Schottky barrier diode that is the first embodiment
of the present invention;
[0017] FIG. 4 is a cross-sectional view describing a method of
fabricating the Schottky barrier diode continued from FIG. 3;
[0018] FIG. 5 is a cross-sectional view describing a method of
fabricating the Schottky barrier diode continued from FIG. 4;
[0019] FIG. 6 is a cross-sectional view describing a method of
fabricating the Schottky barrier diode continued from FIG. 5;
[0020] FIG. 7 is a cross-sectional view describing a method of
fabricating the Schottky barrier diode continued from FIG. 6;
[0021] FIG. 8 is a cross-sectional view describing a method of
fabricating the Schottky barrier diode continued from FIG. 7;
[0022] FIG. 9 is a cross-sectional view showing a first
modification example of the Schottky barrier diode that is the
first embodiment of the present invention;
[0023] FIG. 10 is a cross-sectional view showing a second
modification example of the Schottky barrier diode that is the
first embodiment of the present invention;
[0024] FIG. 11 is a cross-sectional view showing a fabrication
method of a third modification example of the Schottky barrier
diode that is the first embodiment of the present invention;
[0025] FIG. 12 is a cross-sectional view describing the fabrication
method of the Schottky barrier diode continued from FIG. 11;
[0026] FIG. 13 is a cross-sectional view describing the fabrication
method of the Schottky barrier diode continued from FIG. 12;
[0027] FIG. 14 is a circuit diagram of a three-phase motor showing
an application example of the Schottky barrier diode that is the
first embodiment of the present invention;
[0028] FIG. 15 is a cross-sectional view of a Schottky barrier
diode that is a second embodiment of the present invention;
[0029] FIG. 16 is a cross-sectional view of a Schottky barrier
diode shown as a comparison example;
[0030] FIG. 17 is a cross-sectional view of a vertical type diode
shown as a comparison example;
[0031] FIG. 18 is a graph showing characteristics of the vertical
type diode shown as a comparison example; and
[0032] FIG. 19 is a graph showing characteristics of the vertical
type diode shown as a comparison example.
DESCRIPTIONS OF THE PREFERRED EMBODIMENTS
[0033] Hereinafter, embodiments of the present invention will be
described in detail with reference to the accompanying drawings.
Note that components having the same function are denoted by the
same reference symbols in principle throughout the drawings for
describing the embodiment, and the repetitive description thereof
will be omitted.
First Embodiment
[0034] A configuration of a semiconductor device according to a
first embodiment including a Schottky barrier diode will be
described with reference to FIGS. 1 and 2. FIG. 1 is a plan view of
the semiconductor device according to the present embodiment. FIG.
2 is a perspective view showing a partial cutaway view of the
semiconductor device according to the present embodiment. The
cross-sectional view shown in the perspective view of FIG. 2 is a
cross-sectional view taken along the line A-A of FIG. 1.
[0035] As shown in FIGS. 1 and 2, the semiconductor device
according to the present embodiment is formed on a semiconductor
substrate, and is provided with a Schottky barrier diode having a
stack film annularly patterned in plan-view and electrodes formed
at both sides of the stack film, respectively. Specifically, the
semiconductor device according to the present embodiment includes:
a substrate 1; a buffer layer 2, a GaN layer 3, a stack body (stack
structure) 6, a GaN film 6a, and an insulating film 11, all of
which are formed on the substrate 1 in this order from the
substrate 1 side; a first electrode (Schottky electrode) 8 formed
on the sidewall of the inner side of the annular stack structure 6,
and a second electrode (ohmic electrode) 10 formed on the sidewall
of the opposite side thereof. The substrate 1, for example, is a Si
(silicon) substrate. For the substrate 1, a GaN (nitride gallium)
substrate, a SiC (silicon carbide) substrate or a sapphire
substrate or the like may be used in addition to the Si
substrate.
[0036] The buffer layer (buffer layer, stress relaxation layer) 2
is arranged on the substrate 1, and for example, it is a film
composed of an undoped AlGaN layer. The term "undoped" means that
n-type or p-type impurities are not contained or even if the
impurities are contained, the concentration thereof is low. This
buffer layer is formed for the relaxation of the stress arising
from the stacking of the substrate 1 and the GaN film 3 and the
stack structure 6 on the upper part of this substrate 1. The buffer
layer 2 may use an AlN layer and the like in addition to the
undoped AlGaN layer, and a film having a stress reverse to the film
stress (for example, a film warping) generated when stacked may be
used as the buffer layer 2.
[0037] The GaN film 3 is a film formed on the buffer layer 2, and
is integrated with a GaN film 3a formed in the lowest layer of the
stack structure 6 formed on the GaN film 3. That is, one layer film
composed of the GaN films 3 and 3a has a part of the upper part
thereof annularly processed and the lower part thereof not
processed. Here, a region annularly processed is the GaN film 3a,
and a region not annularly processed of the lower part of the GaN
film 3a is the GaN film 3. The GaN film 3a constitutes a part of
the stack structure 6.
[0038] The stack structure 6 has a structure in which semiconductor
films different in forbidden band width are repeatedly stacked. The
semiconductor film constituting the stack structure 6 is a compound
semiconductor, and for example, can use a GaN film that is a
nitride-based compound semiconductor, an
In.sub.xAl.sub.yGa.sub.1-x-yN (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1, 0.ltoreq.x+y.ltoreq.1) film, or the like. As a
specific example, the stack structure 6, for example, can be formed
into a stack structure having the GaN film 3a, an
Al.sub.0.25Ga.sub.0.75N film (barrier film) 4a, a GaN film 3b, and
an Al.sub.0.25Ga.sub.0.75N film (barrier film) 4b in this order
from the lower layer to the upper layer. The members of the GaN
films 3a, 3b may not be GaN films, but may be InAlGaN films.
However, as described below, since the GaN films 3a, 3b are layers
in which electrons two-dimensionally travel, no alloy scattering is
existent, and it is desirable to use the GaN film that can reduce
the on-voltage of the diode. The other InAlGaN films 4a, 4b have no
electrons travelling, and do not function as channel, and
therefore, they are hereinafter simply referred to as barrier films
4a, 4b. The term "alloy scattering" means that crystals
constituting the film have the disturbance (variation) of the
composition, and that travelling (moving) of the electrons inside
the film having the alloy scattering is disturbed as compared with
the film having no alloy scattering inside.
[0039] The GaN film 3a and the barrier film 4a contained in the
stack structure 6 constitute a heterojunction unit 5a, and the GaN
film 3b and the barrier film 4b constitute a heterojunction unit
5b. Here, the composition of an InAlGaN mixed crystal that
constitutes the barrier films 4a, 4b is selected so as to have a
bigger forbidden band width than that of GaN, and, in one
heterojunction unit, the barrier film having a large forbidden band
width is arranged to the upper layer of the GaN layer.
[0040] Thus, inside such the stack structure 6, the pseudo
lattice-matched semiconductor films are joined each other even when
they are different in forbidden width. Here, the stack structure in
which the GaN film and the AlGaN film are alternately stacked is
taken as an example to give an explanation. When GaN and AlGaN are
stacked in a c-axis direction, that is, a direction vertical to a
main surface of the substrate 1, a lattice constant of GaN in the
stacked surface is 0.3189 nm, whereas a lattice constant of AlN is
0.3114 nm, and the lattice constants of GaN and AlN are
approximated to each other. Further, the lattice constant of AlGaN
is a value corresponding to a composition ratio between the lattice
constant of AlN and the lattice constant of GaN, and takes a value
approximate to the lattice constant of GaN. Hence, it is possible
to grow (or deposit) the GaN film and the AlGaN film as a
continuous crystal.
[0041] In such a stack structure in which the GaN film and the
AlGaN film are stacked, an electron layer (channel) is generated in
the vicinity of the interface of the GaN film side due to the
difference in forbidden band width between these films. This
electron layer is referred to also as a two-dimensional electron
gas. For example, in the case of an AlGaN/GaN hetero structure,
since the electron layer (electron gas) having a high concentration
of about 10.sup.13 (cm.sup.-2) in electron density can be obtained,
the on-resistance can be reduced. The stacking of a plurality of
heterojunction units 5a, 5b can further reduce the on-resistance.
When a current flows inside the stack structure 6, in the case of
the film such as the GaN film 3b having its upper surface and lower
surface contacting with the barrier film, the current mainly flows
through the electron layer formed in the vicinity of the upper
surface of the GaN film 3b, and hardly flows in the lower surface
of the GaN film 3b.
[0042] As described above, the GaN film 3a that is a part of the
bottom of the stack structure 6 is integrated with the GaN film 3
of the lower part of the stack structure 6 and the GaN film 3 and
the GaN film 3a constitute one layer. In other words, while the GaN
film 3 extends along the main surface of the substrate 1, a part of
the upper part thereof is annularly patterned, and, in the present
application, this annular pattern is called a GaN film 3a. On the
stack structure 6, the GaN film 6a annularly patterned and the
insulating film 11 are formed similarly to the stack structure 6.
In other words, the GaN film 6a is formed on the barrier film 4b,
and the insulating film 11 is formed on the GaN film 6a. The GaN
film 6a is an undoped layer in which p-type or n-type impurities
are hardly introduced. Further, the sidewall of the center side of
the annular pattern that is the sidewall of the stack structure 6
is defined as a first sidewall 7, and the sidewall of the outside
of the annular pattern that is the other sidewall of the stack
structure 6 is defined as a second sidewall 9. The first sidewall 7
facing the inner side of the annular pattern is formed as being
contacting with a first electrode 8, and the second sidewall 9
opposite to the first sidewall 7 is arranged to contact with the
second electrode 10. The first electrode 8 and the second electrode
10 also contact the respective sidewalls of the GaN film 6a and the
insulating film 11, and also contact the upper surface of the
insulating film 11, respectively. In this way, the first electrode
8 and the second electrode 10 are formed to ride over the upper
part of the insulating film 11. However, the first electrode 8 and
the second electrode 10 are not adjacent to each other, but
electrically insulated.
[0043] The first electrode 8 is continuously formed across the
upper part of the stack structure 6, the first sidewall 7 of the
stack structure 6, and the upper surface of the GaN film 3 inside
the annular pattern of the stack structure 6. Similarly, the second
electrode 10 is continuously formed across the upper part of the
stack structure 6, the second sidewall 9 of the stack structure 6,
and the upper surface of the GaN film 3 outside of the annular
pattern of the stack structure 6. The first electrode 8 is formed
so as to completely cover the upper surface of the GaN film 3
inside the annular pattern of the stack structure 6. As the main
features of the semiconductor device according to the present
embodiment, an oxide film 12 that is an insulating film formed by
oxidizing the semiconductor constituting the barrier films 4a, 4b
is present between the barrier films 4a, 4b constituting the stack
structure 6 and the first electrode 8, respectively. In other
words, the oxide film 12 is interposed between the barrier films
4a, 4b and the first electrode 8, so that the barrier films 4a, 4b
are not directly contacting the first electrode 8.
[0044] The oxide film 12 is a film resulting from oxidation of the
barrier films 4a, 4b composed of AlGaN. The oxide film 12 may be
formed of, for example, silicon oxide, but since it is difficult to
form other films such as the silicon oxide film on the sidewalls
alone of the barrier films 4a, 4b, here, the selective oxidation of
the exposed barrier films 4a, 4b can easily provide the oxide film
12.
[0045] The respective thicknesses of the GaN films 3a, 3b, and the
barrier films 4a, 4b in the direction vertical to the main surface
of the substrate 1 are about 25 nm, and the respective thicknesses
of the heterojunction units 5a, 5b in the same direction are about
50 nm. The thickness of the GaN film 6a on the stack structure 6 in
the same direction is about 2 to 3 nm. Further, the thickness of
the oxide film 12 in the direction along the main surface of the
substrate 1 and in the direction to pass through the center of the
annular stack structure 6, that is, in the direction vertical to
the sidewall of the stack structure 6 is about 1 nm.
[0046] Although the illustration is omitted in FIGS. 1 and 2, an
interlayer dielectric film is formed on the substrate 1 so as to
cover the GaN film 3, the stack structure 6, the insulating film
11, the first electrode 8, and the second electrode 10. Contact
plugs for supplying a specific potential to the first electrode 8
and the second electrode 10 are formed on the first electrode 8 and
the second electrode 10 by penetrating through the interlayer
dielectric film, respectively.
[0047] As described above, the first electrode 8 is a Schottky
electrode to generate a Schottky barrier wall by contacting with
the semiconductor film that constitutes the stack structure 6. The
second electrode 10 is an ohmic electrode that contacts the
semiconductor film constituting the stack structure 6 and has
relatively linear characteristics (ohmic characteristics) in
voltage-current characteristics. As the member of the first
electrode (Schottky electrode) 8, for example, a laminated
electrode of Ni/Au by subsequently stacking Ni (nickel) and Au
(gold), a laminated electrode of Pt/Au or a laminated electrode of
Pd/Au and the like are used. Further, as the member of the second
electrode (ohmic electrode) 10, for example, a laminated electrode
of Ti/Al or the like is used.
[0048] The stack structure 6, the first electrode 8, and the second
electrode 10 constitute a Schottky barrier diode. When a forward
bias voltage is applied to the Schottky barrier diode, electrons
move from the second sidewall 9 to the first sidewall 7 inside the
respective GaN films 3a, 3b and inside the electron layer formed in
the vicinity of the interface in which the upper surfaces of the
GaN films 3a, 3b and the barrier film contact with each other. That
is, the Schottky barrier diode according to the present embodiment
is a semiconductor device having a rectification to prevent the
current from flowing from the first electrode 8 to the second
electrode 10 in forward bias, and to prevent the current from
flowing from the second electrode 10 to the first electrode 8 in
reverse bias. The Schottky barrier diode utilizes a state that
electrons are hardly movable from the first electrode 8 to the GaN
films 3a, 3b as a consequence of the Schottky barrier generated by
a Schottky junction between the first electrode 8 and the GaN films
3a, 3b.
[0049] As shown in FIG. 1, the stack structure 6 and the insulating
film 11 on the stack structure 6 are formed in an annular region on
the substrate 1 (not shown), and the first electrode 8 is formed
inside the annular region, and the second electrode 10 is formed in
another annular region surrounding the annular region. However, the
layouts of the first electrode 8 and the second electrode 10 may be
exchanged. In this case, the oxide film 12 shown in FIG. 2 is
formed not in the first sidewall 7 that is the side surface of the
inner side of the annular region of the stack structure 6, but
formed on the sidewalls of the barrier films 4a, 4b of the second
sidewall 9 side that is the side surface of the outside of the
annular region of the stack structure 6. The shapes of the stack
structure 6, the GaN film 6a thereon, and the insulating film 11 in
plan view are not limited to being circular, but may be polygonal
such as square or hexagonal. It is not limited to annular, and for
example, may be rectangular extending in one direction.
[0050] Next, the effect of the semiconductor device according to
the present embodiment will be described using a comparison
example. FIG. 16 is a cross-sectional view of a Schottky barrier
diode that is a semiconductor device of the comparison example. The
semiconductor device shown in FIG. 16 has the substantially same
structure as that of the semiconductor device according to the
present embodiment described by using FIGS. 1 and 2. That is, a
buffer layer 22 and a GaN film 23 are formed in this order on a
substrate 21, and a stack structure 26 substantially consisting of
a GaN film 23a, a barrier film 24a, a GaN film 23b, and a barrier
film 24b stacked in order from the substrate 21 side is formed on
the GaN film 23. The barrier films 24a, 24b are composed of an
InAlGaN film. The first sidewall 27 that is the other side surface
of the stack structure 6 is formed with a first electrode 28 that
is a Schottky electrode, and a second sidewall 29 that is another
side surface of the stack structure 6 is formed with a second
electrode 30 that is an ohmic electrode. The GaN film 23a and the
barrier film 24a constitute a heterojunction unit 25a, and the GaN
film 23b and the barrier film 24b constitute a heterojunction unit
25b. The heterojunction unit 25a and the heterojunction unit 25b
constitute the stack structure 26.
[0051] However, being different from the semiconductor device
according to the present embodiment, the oxide film 12 (see FIG. 2)
is not formed between the first electrode 28 that is the Schottky
electrode shown in FIG. 16 and the barrier films 24a, 24b. Also,
the GaN film 6a (see FIG. 2) and the insulating film 11 (see FIG.
2) are not formed on the barrier film 24b that constitutes the
stack structure 26. In this manner, when the oxide film 12 is not
provided, but when the first electrode 28 is made to directly
contact the barrier films 24a, 24b that are exposed at the first
sidewall 27 of the stack structure 26, crystal defects remain in
the side surfaces of the barrier films 24a, 24b that are exposed to
dry-etching upon forming the stack structure 6. As a result, a
reverse leakage current is apt to flow between the first electrode
28 and the second electrode 30 through the crystal defects, and a
problem arises in that the breakdown voltage upon reverse bias of
the Schottky barrier diode defined by 1 mA/cm.sup.2 is reduced to
be low as 100.+-.10 V.
[0052] In other words, in the semiconductor device of the
comparison example where the barrier films 24a, 24b and the first
electrode 28 directly contact with each other, the current flows
between the GaN films 23a, 23b and the first electrode 28 not
through the Schottky junction between the GaN films 23a, 23b and
the first electrode 28, but through the region suffered from the
damages of the sidewalls of the barrier films 24a, 24b upon
applying reverse bias voltage. Thus, the breakdown voltage in
reverse bias of the Schottky barrier diode becomes low.
[0053] Note that a reason of generating the reverse leakage current
in this way is that the barrier films stacked on the GaN film
contact with the electrode in a damaged state, whereas a problem
such as the reverse leakage current being generated is avoided in
the structure where the electrode is only Schottky-joined to the
single GaN film. That is, the invention of the present application
is applied to the semiconductor device, which connects a metal
electrode to the sidewall of the heterojunction unit by stacking
and joining the semiconductor film composed of, for example, GaN
and the barrier film composed of, for example, AlGaN.
[0054] As a method for preventing the lowering of a reverse
breakdown voltage of the semiconductor device in this way, the
concentrations of Al inside the barrier films 24a, 24b are
considered to be made lower. Since the higher concentrations of Al
inside the barrier films 24a, 24b are, the easier leakage current
flows, the reverse leakage current can be reduced by suppressing
the concentrations of Al. However, when the barrier films 24a, 24b
whose concentrations of Al are at low levels are formed, a problem
arises in that the electron densities of the electron layers formed
inside the GaN films 23a, 23b are reduced to about 10.sup.12
(cm.sup.-2) and the on-resistance of the Schottky barrier diode is
increased.
[0055] Hence, the present inventors have conducted a preliminary
experiment by using a semiconductor device having a vertical
Schottky barrier diode (hereinafter, referred to as vertical diode)
as shown in FIG. 17.
[0056] FIG. 17 is a cross-sectional view of a semiconductor device
of the comparison example to explain the effect of the
semiconductor device according to the present embodiment. As shown
in FIG. 17, the semiconductor device of the comparison example has
an n.sup.+-type GaN substrate 31 having an n.sup.+-type conductive
type, and has a barrier conductor film 34 on an n.sup.+-type GaN
substrate 31, the composition of the barrier conductor film 34
being graded from an n.sup.--type GaN to an undoped
Al.sub.0.08Ga.sub.0.92N from the lower surface to the upper
surface. In other words, the composition in the vicinity of the
upper surface of the barrier conductor film 34 is composed of
Al.sub.0.08Ga.sub.0.92N, whereas the composition in the vicinity of
the lower surface is composed of n.sup.--type GaN. The barrier
conductor film 34 has a concentration distribution such that the
concentrations of Al vary to higher levels from the lower surface
to the upper surface. A Schottky (anode) electrode 38 containing Pt
(platinum) is formed on the barrier conductor film 34, and an ohmic
(cathode) electrode 40 that is a stack film of Ti and Al films is
formed on the lower surface of the n.sup.+-type GaN substrate 31.
The Schottky electrode 38, for example, has a circular shape in
plan view, and its width that is a length (diameter) of the
Schottky electrode 38 in the direction along the main surface of
the n.sup.+-type GaN substrate 31 is 500 .mu.m. Further, the
thickness in the c-axis c direction of the region mainly containing
undoped Al.sub.0.08Ga.sub.0.92N inside the barrier conductor film
34 is about 50 nm. Although the undoped Al.sub.0.08Ga.sub.0.92N is
a member having a high resistance value, since its thickness is
small, a current flows between the Schottky electrode 38 and the
ohmic electrode 40 due to tunneling effect.
[0057] When such a vertical type diode is formed, before forming
the Schottky electrode 38 on the barrier conductor film 34, the
Schottky electrode 38 and the ohmic electrode 40 were formed after
an UV/O.sub.3 (Ultraviolet/Ozone) treatment was conducted at
200.degree. C. for two hours. Thereby, the vertical type diode was
formed. As a result, the current-voltage characteristics in the
forward direction of the completed vertical type diode were shifted
to a high voltage side by 20 mV as compared to the vertical type
diode formed without being subjected to the UV/O.sub.3 treatment.
The current-voltage characteristic in the forward direction of the
vertical type diode in this situation is shown in FIG. 18.
[0058] FIG. 18 is a graph showing respective current-voltage
characteristics for the vertical type diode irradiating the barrier
conductor film 34 (see FIG. 17) with ultraviolet and the vertical
type diode not irradiating the barrier conductor film 34 with
ultraviolet. The vertical axis shows a forward current (A) and the
horizontal axis shows a forward voltage (V). The graph shown by a
broken line of FIG. 18 shows a current-voltage characteristic of
the vertical type diode in which the barrier conductor film 34 is
not irradiated with ultraviolet, and the graph shown by a solid
line shows a current-voltage characteristic of the vertical type
diode in which the barrier conductor film 34 is irradiated with
ultraviolet. As shown in FIG. 18, the vertical type diode formed
with conducting the UV/O.sub.3 treatment is slightly increased in
the value of the forward voltage in comparison with the vertical
type diode formed without conducting the UV/O.sub.3 treatment.
[0059] The vertical type diode subjected to the UV/O.sub.3
treatment, as shown in FIG. 17, is presumed to be formed with an
oxide film 12a of about 0.1 nm in thickness on the upper surface of
the barrier conductor film 34 through the irradiation of
ultraviolet. In other words, the oxide film 12a in this state is
interposed between the Schottky electrode 38 and the barrier
conductor film 34. Upon measuring respective reverse leakage
currents for the vertical type diode having the oxide film 12a and
the vertical type diode not having the oxide film 12a as described
above, the present inventors have confirmed that, as shown in FIG.
19, the reverse leakage current of the vertical type diode having
the oxide film 12a was reduced by half as compared with that of the
vertical type diode having not the oxide film 12a. FIG. 19 shows
the current-voltage characteristic for the vertical diode having
the oxide film 12a and the vertical diode not having the oxide film
12a. The vertical axis of FIG. 19 shows the reverse current (A) of
the vertical type diode and the horizontal axis shows the reverse
voltage (V) of the vertical type diode. The graph shown by a broken
line shows the characteristics of the reverse leakage current of
the vertical type diode not having the oxide film 12a, and the
graph shown by a solid line shows the characteristics of the
reverse leakage current of the vertical type diode having the oxide
film 12a. As shown in FIG. 19, it is found that the reverse leakage
current of the vertical type diode having the oxide film 12a is
reduced by about half the reverse leakage current of the vertical
type diode not having the oxide film 12a.
[0060] Here, when an oxidation time of the upper surface of the
barrier conductor film 34 by the UV/O.sub.3 treatment is prolonged,
the reverse leakage current can be exponentially reduced. However,
the forward voltage shown in FIG. 18 is rapidly increased as the
oxide film becomes thicker. If the thickness of the oxide film is 1
nm, the increase of the forward voltage for the vertical type diode
formed without performing the UV/O.sub.3 treatment is about 150 mV
and can be suppressed to a virtually negligible degree. Thus, the
optimal range of the thickness of the oxide film 12a can be said to
be 0.1 to 1 nm.
[0061] In the preliminary experiment described with reference to
FIGS. 17 to 19, although the vertical type diode is used, since
ozone and oxygen at the time of the UV/O.sub.3 treatment are
isotropically supplied to the object, even in the horizontal type
diode in which an electrically-conducting path of the Schottky
barrier diode is formed in the direction parallel to the main
surface of the substrate, the forming of the oxide film between the
Schottky electrode and the barrier film can bring the same effect.
In the vertical type diode, since the current flows through a film
having a high resistance value such as the barrier conductor film
34 and the oxide film 12a, the on-resistance of the vertical type
diode is increased. Meanwhile, in the horizontal type diode as
shown in FIG. 2, since the current flows through the GaN films 3a,
3b, the resistance value between the electrodes can be reduced as
compared with the vertical type diode. Further, in the case of
using the horizontal type diode, since the number of electron
layers (channels) can be increased (made to be multichannel) just
by increasing the number of stacking of the heterojunction unit,
the on-resistance of the diode can be easily reduced.
[0062] Consequently, the result of the preliminary experiment is
applied to the semiconductor device according to the present
embodiment, and, as shown in FIG. 2, the oxide film 12 having a
thickness of about 1 nm in the direction along the main surface of
the substrate 1 is provided between the barrier films 4a, 4b
constituting the stack structure 6 and the first electrode 8. In
this manner, when a high voltage is applied to the second electrode
10, that is, when a reverse-bias voltage is applied, the movement
of electrons between the barrier films 4a, 4b and the first
electrode 8 can be suppressed. Therefore, the generation of the
leakage current (hereinafter, referred to as "reverse leakage
current") can be suppressed upon reverse bias as compared with the
case where the oxide film 12 is not formed like the semiconductor
device of the comparison example shown in FIG. 16. This is because
the first electrode 8 and the barrier films 4a, 4b are made not to
directly contact each other through the formation of the oxide film
12, and the reverse leakage current is prevented from flowing due
to crystal defects of the ends of the barrier films 4a, 4b damaged
by dry etching upon patterning the stack structure 6 become the
electrically-conducting paths.
[0063] As a consequence, the semiconductor device according to the
present embodiment eliminates the need for reducing the
concentrations of Al inside the barrier films 24a, 24b for the
purpose of preventing generation of the reverse leakage current. In
the semiconductor device according to the present embodiment, the
electron density of the electron layer formed inside the GaN films
3a, 3b becomes 10.sup.13 (cm.sup.-2), and the electron layer having
a high electron density can be formed as compared with the
semiconductor device of the comparison example described with
reference to FIG. 16, and therefore, it is possible to reduce the
on-resistance of the semiconductor device.
[0064] Further, similarly to the comparison example as shown in
FIG. 16, when the first electrode 28 and the second electrode 30
are formed in a direct contact with the upper surface of the
barrier film 24b composed of the InAlGaN film, an electric field is
concentrated in the interface among the first electrode 28, the
second electrode 30, and the barrier film 24b. This raises a
problem in that the reverse leakage current easily flows between
the first electrode 28 and the second electrode 30 through the
upper surface of the barrier film 24b. In contrast to this, in the
semiconductor device according to the present embodiment, the first
electrode 8 and the second electrode 10 are prevented from being in
a direct contact with the upper surface of the barrier film 4b by
forming the GaN film 6a and the insulating film 11 on the stack
structure 6, so that the reverse leakage current is prevented from
flowing between the first electrode 8 and the second electrode 10.
However, only either one of the GaN film 6 and the insulating film
11 may be formed on the barrier film 4b.
[0065] When only the insulating film 11 is formed on the barrier
film 4b without forming the GaN film 6a, since there is also the
possibility that the reverse leakage current flows between the
first electrode 8 and the second electrode 10 through the
insulating film 11, the insulating film 11 under the region between
the first electrode 8 and the second electrode 10 facing each other
on the barrier film 4b is preferably eliminated. In other words,
the insulating film 11 is formed on the upper surface of the
barrier film 4b and between the first electrode 8 and the second
electrode 10, respectively, whereas the insulating film 11 is
divided into two on the barrier film 4b, and the other interlayer
dielectric film is embedded between the patterns of the two
insulating films 11 formed on the barrier film 4b.
[0066] Further, it is considered that the Schottky barrier diode
may form a Schottky electrode on the main surface of the
semiconductor substrate having an n-type conductive type through
the barrier film composed of undoped AlGaN or the like, and also
may form the Schottky electrode as the vertical type diode that
forms the ohmic electrode on the rear surface opposite to the main
surface of the semiconductor substrate. However, in this structure,
when the current flows in the direction vertical to the main
surface of the semiconductor substrate, since the current flows
through the barrier film having a high resistance value, a problem
arises in that the on-resistance of the Schottky barrier diode is
increased. In contrast to this, in the semiconductor device
according to the present embodiment, since the first electrode 8
and the second electrode 10 are brought into contact with the
sidewalls of the stack structure 6, the current can be let flow
without involving a barrier film upon forward bias, and a low
resistance junction can be realized.
[0067] When the pattern of the stack structure 6 is formed not in
an annular shape, but in a rectangular shape extending in one
direction along the main surface of the substrate 1, a special
structure for mitigating an electric field is necessary to be
provided in order to prevent the field concentration from occurring
in the end of the pattern. In contrast to this, the end of the
pattern is prevented from being formed by making the pattern of the
stack structure 6 annular as shown in FIGS. 1 and 2, and thus the
electric field concentration can be prevented from occurring in a
part of the pattern. This makes the structure of the Schottky
barrier diode simple and can simplify the fabrication process.
[0068] Since the Schottky barrier diode is a semiconductor element
that functions as a diode by bringing the first electrode 8 that is
the Schottky electrode shown in FIG. 2 into contact with the
heterojunction units 5a, 5b, there is no problem when the second
electrode 10 contacts the barrier films 4a, 4b damaged in the
second sidewall 9. When the oxide film is formed between the second
electrode 10 and the barrier films 4a, 4b in the second sidewall 9,
there is a possibility that the on-resistance is increased. Hence,
even if the oxide film 12 is provided at the first sidewall 7 side,
it is desirable that an oxide film is not formed on the second
sidewall 9 side. Hence, in the semiconductor device according to
the present embodiment, although an oxide film is not provided
between the second electrode 10 and the barrier films 4a, 4b, when
the increase of the on-resistance does not cause a problem, an
oxide film may be formed between the second electrode 10 and the
barrier films 4a, 4b in the second sidewall 9 similarly to the
oxide film 12.
[0069] Next, the fabrication method of the semiconductor device
according to the present embodiment will be described with
reference to FIGS. 3 to 8, and at the same time, the configuration
of the semiconductor device will be more clarified. FIGS. 3 to 8
are cross-sectional views Showing the fabricating process of the
semiconductor device according to the present embodiment, and show
a cross-sectional view at the same position as that of the
cross-sectional view of FIG. 2.
[0070] First, as shown in FIG. 3, for example, a Si substrate is
prepared as the substrate 1, and for example, an AlGaN layer is
formed on the substrate 1 as the buffer layer 2 in the thickness of
about 2 .mu.m by using a vapor phase epitaxy method. Here, impurity
compounds are not introduced into the film-forming apparatus, and
the buffer layer 2 is used as an undoped layer. Subsequently, the
GaN film 3 is formed on the buffer layer 2 as a compound
semiconductor film in the thickness of about 2 .mu.m by using the
vapor phase epitaxy method.
[0071] Subsequently, the barrier film 4a composed of an
Al.sub.0.25Ga.sub.0.75N film is formed on the GaN film 3 in the
thickness of about 25 nm by using the vapor phase epitaxy method.
After that, a GaN film 3b is formed on the barrier film 4a in the
thickness of about 25 nm, and then, the barrier film 4b composed of
the Al.sub.0.25Ga.sub.0.75N film is formed on the GaN film 3b in
the thickness of about 25 nm. As described above, the GaN film and
the Al.sub.0.25Ga.sub.0.75N film are approximated to each other in
lattice constant, and can be formed as a continuous crystal just by
adjusting a source gas in the vapor phase epitaxy method. After
that, the GaN film 6a that is an undoped layer is formed on the
barrier film 4b by using the vapor phase epitaxy method. Then, as
the insulating film 11, for example, a silicon oxide film is
deposited on the GaN film 6a by a CVD (Chemical Vapor Deposition)
method.
[0072] Next, as shown in FIG. 4, a photoresist film (not shown) is
formed on the insulating film 11, and is exposed and developed by
using photolithography technology, thereby allowing the photoresist
film to remain in a prescribed region. Subsequently, with the
remaining photoresist film used as a mask, the insulating film 11
is processed by dry etching so as to form a hole from which the
upper surface of the GaN film 6a is exposed, and then, the
photoresist film is removed. Hereinafter, such a process is
referred to as patterning, where a film having a prescribed shape
(for example, a photoresist film) is formed and the other film is
etched (selectively removed) with that film used as a mask, and
then, a pattern having a desired shape is formed. By this
patterning process, the insulating film 11 is formed in a desired
region. Here, by removing the insulating film 11 in a circular
region in plan view, a through-hole that penetrates the insulating
film 11 is formed.
[0073] After that, with the remaining insulating film 11 used as a
mask, the stack film of the GaN film 6a, the barrier film 4b, the
GaN film 3b, the barrier film 4a, and the GaN film 3, all of which
are located immediately below the through-hole, is dry-etched down
to a prescribed depth from the upper surface of the lowermost GaN
film 3, and the first sidewall 7 (see FIG. 2) that is the inner
sidewall of the stack structure 6 (see FIG. 2) formed by a
subsequent process is formed. In other words, here, the stack film
is removed by etching down to the middle of the depth of the GaN
film 3, and the buffer layer 2 is not allowed to be exposed. In the
dry etching of this process, for example, a dry etching using, for
example, chlorine plasma is used. Here, the side surfaces of the
barrier films 4a, 4b exposed by the dry etching process is damaged
by the dry etching. Thus, insulation properties are considered to
be deteriorated because crystal defects are generated as a
result.
[0074] Next, as shown in FIG. 5, a specimen including the substrate
1 and a structure formed thereon is heated to 200.degree. C., and
is radiated with ozone generated by a mercury lamp having a
wavelength of 184.9 nm for two hours. Upon this ozone radiation
treatment (UV/O.sub.3 treatment), the side surfaces of the GaN
films 3, 3b and 6a and the upper surface of the GaN film 3 exposed
by dry etching are hardly deteriorated, whereas the side surfaces
of the exposed barrier films 4a, 4b are oxidized, and the oxide
film 12 having a main component of Al.sub.2O.sub.3 and the
thickness in the direction along the main surface of the substrate
1 of about 1 nm is selectively formed. In other words, the oxide
film 12 is an oxide of the barrier films 4a, 4b.
[0075] Next, as shown in FIG. 6, the photoresist film is made to
remain in the prescribed region on the insulting film 11 by
photolithography technology, and then, with the photoresist film
used as a mask, the insulating film 11 is etched, and after that,
the photoresist film is removed. Therefore, the insulating film 11
having an annular shape in plan view is formed. Then, with the
remaining insulating film 11 used as a mask, the stack film of the
GaN film 6a, the barrier film 4b, the GaN film 3b, the barrier film
4a, and the GaN film 3 is dry-etched down to the prescribed depth
from the upper surface of the lowermost GaN film 3, and the second
sidewall 9 that is the sidewall outside of the stack structure 6 is
formed. In other words, here, the stack film is removed by etching
down to the middle of the depth of the GaN film 3, and the buffer
layer 2 is not allowed to be exposed. Here, for example, a dry
etching using chlorine plasma is performed.
[0076] By this process, the GaN film 3a is formed, which is the
upper region inside the GaN film 3 whose upper part is partially
removed and which is a projection having a sidewall of curved
surface and composed of the GaN film 3. The GaN film 3 and the GaN
film 3a are an integrated layer, and the GaN film 3a is a pattern
having an annular shape in plan view. By the process as described
above, the stack structure 6 composed of a plurality of annular
patterns sequentially stacked on the GaN film 3 is formed. The
stack structure 6 is composed of the heterojunction unit 5a and the
heterojunction unit 5b on the heterojunction unit 5a. The
heterojunction unit 5a is composed of the GaN film 3a and barrier
film 4a on the GaN film 3a, and the heterojunction unit 5b is
composed of the GaN film 3b and the barrier film 4b on the GaN film
3b. Consequently, the stack structure 6 includes the GaN film 3a,
the barrier film 4a, and the GaN film 3b and the barrier film 4b
sequentially stacked on the GaN film 3, and has an annular shape in
plan view.
[0077] Further, the GaN film 6a and the insulating film 11, which
are annular patterns, are sequentially stacked on the upper part of
the stack structure 6 from the stack structure 6 side. The sidewall
of the inner side of the stack structure 6 that is the annular
pattern is the first sidewall 7, and the sidewall of the outside
that is the other sidewall is the second sidewall 9. Here, the
respective sidewalls of the barrier films 4a, 4b of the first
sidewall 7 side are formed with the oxide film 12, whereas the
respective sidewalls of the barrier films 4a, 4b of the second
sidewall 9 side are not formed with the oxide film 12. The side
surfaces of the barrier films 4a, 4b are exposed at the second
sidewall 9.
[0078] Next, as shown in FIG. 7, the photoresist film is formed by
using photolithography technology so as to cover the periphery part
of the upper surface of the insulating film 11, the second sidewall
9, and the upper surface of the GaN film 3 of the region outside
the stack structure 6. More specifically, the outer periphery part
in the vicinity of the insulating film 11 that is the annular
pattern is covered by the photoresist film, whereas the inner
periphery part of the insulating film 11, the first sidewall 7 of
the inner side of the inner periphery part, and the upper surface
of the GaN film 3 are exposed from the photoresist film.
Subsequently, a metal film as an electrically conductive film is
deposited on the entire surface of the main surface of the
substrate 1 as well as on the photoresist film. The metal film, for
example, is a stacked film of Ni/Au stacked with Ni and Au in this
order from the substrate 1 side, and is formed by using a
sputtering method, an EB (Electron Beam) evaporating method or the
like.
[0079] Subsequently, the stacked film of Ni/Au is made to remain in
the desired region only by technique (lift-off process) of removing
the photoresist film together with the stacked film of Ni/Au
deposited thereon, and the first electrode 8 composed of the
stacked film of Ni/Au that contacts the first sidewall 7 of the
stack structure 6 and the oxide film 12 is formed. In other words,
the first electrode 8 is continuously formed with the upper surface
of the GaN film 3 of the inner side of the first annular sidewall
7, the first sidewall 7, the sidewall of the inner side of the GaN
film 6a, the sidewall of the inner side of the insulating film 11,
and the upper surface of the vicinity of the inner periphery part
of the insulating film 11. The first electrode 8 is a Schottky
electrode that forms a Schottky junction with the GaN films 3a, 3b
at the first sidewall 7. Since the first sidewall 7 is formed with
the oxide film 12, the first electrode 8 contacts the oxide film 12
but does not contact the barrier films 4a, 4b. Therefore, the first
electrode 8 does not contact the sidewalls of the damaged barrier
films 4a, 4b.
[0080] Next, as shown in FIG. 8, the process similar to the process
described with reference to FIG. 7 is performed, and the second
electrode 10 (for example, Ti/Au) contacting the second sidewall of
the stack structure 6 is formed by using the lift-off method. Here,
the second electrode 10 is formed not inside of the stack structure
6 that is the annual pattern, but only on the upper surface of the
GaN film 3 of the region outside of the stack structure 6, the
second sidewall 9, the sidewall outside of the GaN film 6a, the
sidewall of the outside of the insulating film 11, and the upper
surface of the outer periphery part of the insulating film 11. In
this manner, the second electrode 10 contacts the barrier films 4a,
4b at the second sidewall 9, and is also ohmic-connected to the GaN
films 3a, 3b. One end of the upper surface of the insulating film
11 is covered by the first electrode 8, and the other end on the
opposite side is covered by the second electrode 10. Since the
insulating film 11 is exposed between the first electrode 8 and the
second electrode 10, the first electrode 8 and the second electrode
10 do not contact each other, and are electrically isolated. Thus,
the Schottky barrier diode having the stack structure 6, the first
electrode 8, and the second electrode 10 is formed.
[0081] Although the subsequent process is not illustrated, a thick
interlayer dielectric film is formed on the substrate 1 so as to
cover the stack structure 6, the first electrode 8, and the second
electrode 10 by a CVD method or the like, and then, contact plugs
that penetrate through the interlayer dielectric film are formed on
the first electrode 8 and the second electrode 10. Subsequently, a
plurality of metal wirings are formed on the contact plugs, and the
plurality of metal wirings, the first electrode 8, and the second
electrode 10 are electrically connected through the contact plugs,
respectively, thereby completing the semiconductor device according
to the present embodiment.
[0082] In the above-described fabrication process, the
semiconductor device is formed in the order of the following
processes: "the formation of the first sidewall, the oxidation of
the barrier films, the formation of the second sidewall, the
formation of the first electrode, and the formation of the second
electrode". However, this process order can be changed to either
"the formation of the first sidewall, the oxidation of the barrier
films, the formation of the first electrode, the formation of the
second sidewall, and the formation of the second electrode" or "the
formation of the second sidewall, the formation of the second
electrode, the formation of the first sidewall, the oxidation of
the barrier films, and the formation of the first electrode".
[0083] In the above-described fabrication process, although the
insulating film 11 is formed on the upper side of the stack
structure 6, if the GaN film 6a is formed on the stack structure 6,
the insulating film 11 may not be formed. In this case, the
patterning of the GaN film 6a and the stack structure 6 does not
use the insulating film 11 as a mask but uses the photoresist film
as a mask. Alternatively, with using the insulating film 11 as a
mask, the patterning of the GaN film 6a and the stack structure 6
is performed, and then, the insulating film 11 is removed.
[0084] As described above, when the first electrode 8 and the
second electrode 10 directly contact the upper surface of the
barrier film 4b of the upper side of the stack structure 6, this
becomes a cause of generating the reverse leakage current.
Therefore, in view of preventing generation of the reverse leakage
current, it is desirable to form the insulating film 11 on the
stack structure 6, if the GaN film 6a is not formed. When the GaN
film 6a is not formed, the insulating film 11 is formed directly on
the upper surface of the barrier film 4b. This leads to the
possibility of making the reverse leakage current to flow between
the first electrode 8 and the second electrode 10 through the
insulating film 11.
[0085] The generation of such a reverse leakage current is caused
by incompatibility between the insulating film 11 and the AlGaN
film. Consequently, similarly to the semiconductor device shown in
FIG. 2, when the insulating film 11 is formed on the AlGaN film via
the GaN film 6a, there is no problem even if the insulating film 11
is continuously formed between the first electrode 8 and the second
electrode 10. The insulating film 11 shown in FIG. 2 is a film
simply provided for the purpose of being used as a patterning mask
of the stack structure 6, and is not provided for the purpose of
preventing the first electrode 8 and the second electrode 10 from
directly contacting the barrier film 4b. In the semiconductor
device shown in FIG. 2, it is the GaN film 6a that prevents the
first electrode 8 and the second electrode 10 from directly
contacting the barrier film 4b.
[0086] In contrast to this, when the GaN film 6a is not formed, it
is desirable that the insulating film 11 directly below the region
between the first electrode 8 and the second electrode 10 is
removed with the insulating film 11 directly below the first
electrode 8 and the second electrode 10 left alone in order to
prevent the reverse leakage current from flowing through the
interface between the insulating film 11 and the barrier film 4b,
thereby to expose the upper surface of the barrier film 4b as shown
in FIG. 9. In other words, the insulating film 11 directly below
the first electrode 8 and the insulating film 11 directly below the
second electrode 10 are not formed continuously but divided so that
the reverse leakage current can be prevented from flowing through
the interface of the insulating film 11 and the barrier film 4b.
FIG. 9 shows a cross-sectional view of the same position as that of
the cross-sectional view of FIG. 2.
[0087] FIG. 9 is a cross-sectional view of the semiconductor device
of a first modification example according to the present embodiment
having substantially the same structure as that of the
semiconductor device shown in FIG. 2. However, being different from
the semiconductor device shown in FIG. 2, the GaN film 6a is not
formed on the stack structure 6 of the semiconductor device shown
in FIG. 9, and the insulating film 11 is not formed immediately
below the region between the first electrode 8 and the second
electrode 10 on the insulating film 11, and a part of the upper
surface of the barrier film 4b is exposed from the insulating film
11.
[0088] Meanwhile, even when the GaN film 6a and the insulating firm
11 are not formed, it is considered that the oxide film 12 that is
the insulating film is formed at the side surfaces of the barrier
films 4a, 4b of the first sidewall 7 by the oxidation process based
on the UV/O.sub.3 treatment described with reference to FIG. 5, and
the oxide film 12 is also formed on the upper surface of the
exposed barrier film 4b. In this case, even if the first electrode
8 and the second electrode 10 are formed on the barrier film 4b
after performing the process described with reference to FIG. 5,
the reverse leakage current can be prevented from flowing through
the upper surface of the barrier film 4b because the oxide film 12
is formed between the barrier film 4b and the first electrode 8 and
the barrier film 4b and the second electrode 10 as shown in FIG.
10.
[0089] FIG. 10 is a cross-sectional view of the semiconductor
device of a second modification example according to the present
embodiment, which has substantially the same structure as that of
the semiconductor device shown in FIG. 2 and shows a
cross-sectional view of the same position as that of the
cross-sectional view of FIG. 2. However, being different from the
semiconductor device shown in FIG. 2, the GaN film 6a and the
insulating film 11 are not formed on the stack structure 6 of the
semiconductor device shown in FIG. 10, and instead, the oxide film
12 covers the upper surface of the barrier film 4b. The thickness
of the oxide film 12 of the upper surface of the barrier film 4b is
about 1 nm in the direction vertical to the main surface of the
substrate 1.
[0090] The reason why the GaN film 6a, the insulating film 11 or
the oxide film 12 are provided on the upper surface of the stack
structure 6 as described above is to prevent the generation of the
reverse leakage current caused by the formation of the first
electrode 8 and the second electrode 10 by riding over the upper
surface of the stack structure 6. Therefore, when the first
electrode 8 and the second electrode 10 are not formed on the stack
structure 6, but the first electrode 8 and the second electrode 10
are formed at the sidewall of the stack structure 6, there is no
need to form the GaN film 6a, the insulating film 11 or the oxide
film 12 on the upper surface of the stack structure 6.
[0091] As described above, in the semiconductor device of the
comparison example described with reference to FIG. 16, there is a
problem in that, because the barrier films 24a, 24b directly
contact the first electrode 28, the reverse leakage current flows
between the first electrode 28 and the GaN films 23a, 23b through
the sidewalls of the barrier films 24a, 24b that are exposed by dry
etching and left with crystal defects. In this case, the breakdown
voltage of the diode that is defined to be 1 mA/cm.sup.2 becomes
100.+-.10 V.
[0092] In contrast to this, according to the investigations
conducted by the present inventors, when the reverse breakdown
voltage of the Schottky barrier diode in the semiconductor device
according to the present embodiment described with reference to
FIGS. 3 to 10 was measured, a good result of 800.+-.100V was
obtained. This is because the reverse leakage current can be
prevented from flowing by providing the oxide film 12 between the
first electrode 8 and the barrier films 4a, 4b as shown in FIG. 8
even if the crystal defects due to the dry etching at the side
surface of the barrier films 4a, 4b remain.
[0093] The composition of the barrier films 4a, 4b may be the AlGaN
film having a composition different from the above-described
composition, and even if the AlGaN film having a composition
different from the above described composition is used, the same
effect as described above can be obtained.
[0094] While the heterojunction unit is staked in two layers in the
structure of the semiconductor device as described above, three or
more layers of the heterojunction unit may be stacked. The increase
in the number of layers of the heterojunction unit, that is, the
number of channels in this way can reduce the on-resistance of the
Schottky barrier diode.
[0095] The above-described fabrication process uses a method for
oxidizing the surfaces of the barrier films 4a, 4b exposed at the
first sidewall 7 before forming the second sidewall 9 or in a state
in which the second sidewall 9 is not exposed so as to prevent the
oxide film from being formed between the second electrode 10 that
is the ohmic electrode and the barrier films 4a, 4b. As a third
modification example, in contrast to this, a fabrication method for
forming the stack structure 6 by one etching process will be
described below with reference to FIGS. 11 to 13. FIGS. 11 to 13
show cross-sectional views of the same position as that of the
cross-sectional view of FIG. 2.
[0096] In the fabrication process of the semiconductor device that
is the third modification example, first, the stack structure shown
in FIG. 3 is formed, and then, as shown in FIG. 11, a pattern of
the stack structure 6 is formed by using the photolithography
technology and the dry etching method. Here, the structure of the
semiconductor device in the midst of the fabrication process is
almost the same as the structure shown in FIG. 6 except that the
oxide film 12 is not formed, but different from the fabrication
method as described above in that the side surfaces of the barrier
films 4a, 4b are exposed at the first sidewall 7. In this case,
crystal defects are considered to be generated as the side surfaces
of the barrier films 4a, 4b of the first sidewall 7 side are
damaged by the dry etching.
[0097] Next, an insulating film 13 composed of, for example, a
silicon oxide film is formed on the entire surface of the main
surface of the substrate 1 so as to cover the stack structure 6 by
the CVD method and the like, and then, the insulating film 13 is
processed by the photolithography technology and the dry etching
method. This exposes the first sidewall 7 of the stack structure 6
and the upper surface of the GaN film 3 of the inner side of the
first sidewall 7. Subsequently, the oxide film 12 is formed on the
side surfaces of the barrier films 4a, 4b, that is, only on the
side surfaces 4a, 4b on the first sidewall 7 side by performing the
UV/O.sub.3 treatment.
[0098] Next, although the insulating film 13 is removed, in order
to prevent the oxide film 12 from being removed by an etchant
(etching solution) such as hydrofluoric acid used here, an
insulating film 14 composed of, for example, a silicon nitride film
is formed on the entire surface of the main surface of the
substrate 1 so as to cover the stack structure 6 and the insulating
film 13 by the CVD method and the like, and then, the insulating
film 13 is processed by using the photolithography technology and
the dry etching method. Therefore, an insulating film 14 that
covers the insulating film 12 and exposes the insulating film 13 is
formed.
[0099] After that, though not illustrated, the insulating film 13
is removed by wet etching using hydrofluoric acid, and then, the
insulating film 14 is removed by wet etching using phosphoric acid,
so that the same structure as that of FIG. 6 can be obtained. In
the succeeding processes, the processes described with reference to
FIGS. 7 to 8 are performed, thereby completing the semiconductor
device of the third modification example.
[0100] In the semiconductor device of the third modification
example thus formed, since the sidewalls of both sides of the stack
structure 6 are patterned by one etching process, variations in the
distance between the first sidewall 7 and the second sidewall 9 due
to the misalignment of a photo mask can be prevented from
occurring. In other words, the semiconductor device can be formed
by maintaining the distance between the anode electrode and the
cathode electrode constant, and therefore, variations in breakdown
voltage of the semiconductor device can be suppressed.
[0101] In the fabrication method described with reference to FIGS.
3 to 8, the stack structure 6 is formed by two times of etching so
that an oxide film is not formed between the second electrode 10
and the barrier films 4a, 4b, and oxidation of respective side
surfaces of the barrier films 4a, 4b is performed after the first
sidewall 7 is formed and before the second sidewall 9 is formed.
Further, in the fabrication method described by using FIGS. 11 to
13, the oxidation of respective side surfaces of the barrier films
4a, 4b is performed in a state in which the insulating film 13 is
formed for the purpose of not forming the oxide film similarly
between the second electrode 10 and the barrier films 4a, 4b.
[0102] However, when the increase of the on-resistance due to the
formation of the oxide film between the second electrode 10 and the
barrier films 4a, 4b is to such an extent of not causing a problem,
an oxide film may be formed between the second electrode 10 and the
barrier films 4a, 4b in the second sidewall 9 similarly to the
oxide film 12. In that case, the fabrication method described with
reference to FIGS. 3 to 8 and FIGS. 11 to 13 is not employed, but
for example, the structure having the stack structure 6 is formed
in a lump by the etching process described with reference to FIG.
11, and then, the respective side surfaces of both sides of the
barrier films 4a, 4b are oxidized. After that, the Schottky barrier
diode is considered to be formed by the processes of FIGS. 7 and
8.
[0103] An application example of the Schottky barrier diode
described with reference to FIGS. 1 to 13 will be described below.
The semiconductor device according to the present embodiment can be
used, for example, for a drive circuit of a three-phase motor used
in a hybrid vehicle and so forth.
[0104] FIG. 14 is a circuit diagram of a three-phase motor using
the Schottky barrier diode according to the present embodiment. As
shown in FIG. 14, the three phase-motor circuit has a three-phase
motor 41, a power semiconductor device 42, and a control circuit
43. The three-phase motor 41 is constituted in such a manner as to
be driven by three-phase voltages different in phase. The power
semiconductor device 42 being shown surrounded by a broken line in
FIG. 14 is constituted by a switching element for controlling the
three-phase motor 41, and for example, an IGBT (Insulated Gate
Bipolar Transistor) 44 and a diode 45 are provided corresponding to
each of the three phases. That is, in each single phase out of the
three phases, the IGBT 44 and the diode 45 are connected in
reverse-parallel between a power-supply voltage (Vcc) and an input
voltage of the three-phase motor. The IGBT 44 and the diode 45 are
also connected in reverse-parallel between the input voltage of the
three-phase motor and a ground potential (GND). In other words, the
three-phase motor 41 is provided with two IGBTs 44 and two diodes
45 at every single phase (each phase), and therefore, six IGBTs 44
and six diodes 45 are provided in three phases.
[0105] Gate electrodes of the respective IGBTs 44, although the
illustration thereof is partially omitted, are connected to the
control circuit 43, respectively, and by this gate control circuit
43, the IGBTs 44 are controlled. In the drive circuit of the
three-phase motor having such a power semiconductor device 42 and a
control circuit 43, the three-phase motor 41 is rotated by
controlling the current flowing through the IGBTs 44 (switching
element) constituting the power semiconductor device 42 by the
control circuit 43. In other words, the IGBTs 44 function as
switching devices for supplying the power supply potential (Vcc) to
the three-phase motor 41 or for supplying the ground voltage (GND),
and the three-phase motor 41 can be driven by controlling on/off
timing of the IGBTs 44 by the control circuit 43.
[0106] The IGBT 44 and the diode 45, as shown in FIG. 14, are
connected in reverse-parallel to each other, and the function of
the diode 45 here will be described below.
[0107] When there is a load that is purely resistive and does not
contain inductance, no free-wheeling energy is induced, and
therefore, the diode 45 is not required. However, when a load is
connected with a circuit containing inductance such as a motor (for
example, three-phase motor), there is a mode wherein a load current
flows in the direction reverse to a turned-on switch (IGBT 44).
Here, since a single element of the switching device is not
provided with a function capable of allowing this reverse current
to flow, a diode is necessary to be connected in reverse-parallel
to the switching device such as the IGBT 44. That is, in an
inverter circuit, when inductance is contained in the load like in
motor control, the energy stored in inductance must be surely
discharged in the case that the switching device such as the IGBT
44 is turned off. The single element of the IGBT 44 is unable to
flow the reverse current for discharging the energy stored in
inductance. Hence, in order to flow back the electrical energy
stored in the inductance, the IGBT 44 is connected in
reverse-parallel to the diode 45. In other words, the diode 45 has
a function of letting flow the reverse current to discharge the
electrical energy stored in the inductance.
[0108] When the Schottky barrier diode described in the present
embodiment is used as such a diode 45, the circuit performance of
the three-phase motor can be improved. The three-phase motor
circuit is just only one of application examples of the
semiconductor device according to the present embodiment, and it is
needless to say that the semiconductor device according to the
present embodiment can be used in a variety of circuits.
Second Embodiment
[0109] While the stacked film of the GaN film and the AlGaN film
has been used as a heterojunction unit in the first embodiment, the
heterojunction unit stacked with other semiconductor films may also
be used. For example, instead of the AlGaN film, an InAlN film may
be used. That is, as the heterojunction unit, a stack film of a GaN
film and an InAlN film may be used. As described above, the lattice
constant of GaN in the stack layer surface which is the interface
between the films constituting the heterojunction unit is 0.3189
nm, the lattice constant of AlN is 0.3114 nm, and the lattice
constant of InN is 0.3548 nm. Thus, the lattice constant of InGaN
has a value corresponding to a composition ratio between the
lattice constant of AlN and the lattice constant of InN, and takes
a value approximate to the lattice constant of GaN. Here, the
forbidden band width of GaN is smaller than that of InGaN, and in
one heterojunction unit, the InAlN film which is a film great in
forbidden band width is arranged at an upper layer.
[0110] With the heterojunction unit of the GaN film and the InAlN
film used in this way, the heterojunction units of about 100 pairs
are stacked to enable the number of channels to be substantially
increased, so that the on-resistance can be further reduced.
[0111] A cross-sectional view of the semiconductor device according
to the present embodiment is shown in FIG. 15. A Schottky barrier
diode shown in FIG. 15 has the same structure as that of the
semiconductor device according to the first embodiment shown in
FIG. 8, whereas, as shown in FIG. 15, barrier films 40a, 40b
composed of InAlN film instead of AlGaN film are formed on
respective upper parts of the GaN films 3a, 3b. In other words, the
semiconductor device according to the present embodiment is
different in the member of the barrier film from the semiconductor
device according to the first embodiment.
[0112] Further, heterojunction units 5c, 5d are formed between the
barrier film 40b corresponding to the barrier film 4b (see FIG. 8)
and the GaN film 6 in order from the substrate 1 side, and the way
the heterojunction units 5a to 5c form the stack structure 6 is
also different from the first embodiment. In other words, a GaN
film 3c and a barrier film 40c stacked on the barrier film 40b in
this order constitute the heterojunction unit 5c, and a GaN film 3d
and a barrier film 40d stacked on the heterojunction unit 5c in
this order constitute the heterojunction unit 5d. This shows that a
number of heterojunction units can be stacked in the semiconductor
device according to the present embodiment as compared with the
semiconductor device according to the first embodiment. In FIG. 15,
to make the illustration easy to understand, the heterojunction
units 5a to 5d are shown in four layers. However, in practice,
stacking of the heterojunction units for about 100 layers may be
performed. The oxide films 12 are formed at the interfaces between
the respective barrier films 40a to 40d and the first electrode
8.
[0113] When the heterojunction unit is constituted by the stacked
film of the GaN film and the AlGaN film formed on the GaN film, in
the case that a ratio of Al inside the AlGaN film is made larger
than 0.25 due to a difference in lattice constant between GaN and
AlGaN, the stress inside the heterojunction is increased, and there
is a high possibility that a crack develops in the heterojunction
unit. The stress becomes larger as the number of stack layers of
the heterojunction unit that constitutes the stack structure 6
increases. Consequently, in the Schottky barrier diode stacked with
the heterojunction unit composed of the GaN film and the AlGaN
film, the crack easily develops when a number of heterojunction
units are stacked. Therefore, the number of heterojunction units to
be stacked needs to be limited to about three. In other words, in
view of securing the reliability of the semiconductor device, when
the heterojunction unit is constituted by the stack film of the GaN
film and the AlGaN film formed on the GaN film, the number of
heterojunction units that can be stacked are limited to about two
or three layers.
[0114] In contrast to this, similarly to the semiconductor device
according to the present embodiment, when the heterojunction unit
of the GaN film and the InAlN film is used, the lattice constants
of GaN and InAlN are easily equalized, and the stress is hardly
generated in the heterojunction unit. Therefore, for example, the
stack structure 6 can be formed by a plurality of heterojunction
units which are stacked for 1000 layers. Since the thicknesses of
the GaN film and InAlN film that constitute the heterojunction unit
are about 25 nm, respectively, the thickness of one layer of
heterojunction unit is about 50 nm. As described above, the
heterojunction unit can be stacked for 1000 layers by preventing
the crack from developing, but in the semiconductor device actually
used, it is considered, for example, that the heterojunction unit
is stacked for 100 layers and the stack structure 6 having the
thickness of about 5 .mu.m is formed.
[0115] In the semiconductor device according to the present
embodiment, a large number of heterojunction units can be formed as
compared with the semiconductor device using the heterojunction
unit composed of the GaN film and the AlGaN film. Since the number
of electron layers (channels) in which electrons pass between the
first electrode 8 and the second electrode 10 can be formed more as
the number of heterojunction units inside the stack structure 6 are
larger, the on-resistance can be reduced.
[0116] The semiconductor device according to the present embodiment
can be formed by the same process as that of the first embodiment.
However, the AlN molar ratio of the barrier film of the first
embodiment is about 0.25, while the AlN molar ratio of the
semiconductor device according to the present embodiment is about
0.8 and relatively high. Thus, when the oxide film 12 of the first
embodiment is formed, ozone radiation is performed, while the oxide
film of the barrier film can be formed also by wet oxidation in
water-vapor atmosphere in the present embodiment. This is because
the barrier film is hardly oxidized by oxidation (wet oxidation)
when the Al concentrations of the barrier film are relatively low
to a level of about 30%, but when the Al concentrations of the
barrier film are relatively high to a level of 70% or more, the
surface of the barrier film can be easily oxidized by
wet-oxidation. Note that, the respective side surfaces of the GaN
films 3a, 3b and the surface of the dry-etched GaN film 3 are
hardly deteriorated. That is, upon oxidizing the barrier film, the
damages posed on the GaN films can be reduced not by performing
ozone radiation using ultraviolet but by performing wet
oxidization.
[0117] Although the semiconductor device of the comparison example
explained with reference to FIG. 16 has a reverse breakdown voltage
of 100.+-.10V, the semiconductor device according to the present
embodiment has a reverse breakdown voltage of 800.+-.100V of the
Schottky barrier diode and can obtain a good reverse breakdown
voltage as compared with the semiconductor device of the comparison
example. Further, the suppression of the crack development inside
the heterojunction unit can allow the heterojunction unit to be
stacked for, for example, 100 layers, and can increase the number
of channels largely, so that the on-resistance can be reduced.
[0118] Consequently, the semiconductor device according to the
present embodiment can obtain the effect of reducing the reverse
leakage current of the diode similarly to the effect of the first
embodiment, and can reduce the on-resistance of the Schottky
barrier diode by allowing the heterojunction unit to be staked much
more.
[0119] In the foregoing, the invention made by the inventors of the
present invention has been concretely described based on the
embodiments. However, it is needless to say that the present
invention is not limited to the foregoing embodiments and various
modifications and alterations can be made within the scope of the
present invention.
[0120] The fabrication method of the semiconductor device according
to the present invention can be widely used for semiconductor
devices having heterojunction units different in forbidden band
width.
* * * * *